Many systems are known, which involve gas/liquid contact, e.g. for the aerobic digestion of biomass, using oxygen or air, and for the disinfection of water, using chlorine or, increasingly, ozone. These systems are generally based on the principle of introducing bubbles of the gas into the liquid, e.g. in counter-current, with a view to maximising the degree of contact between the active gas and the liquid.
A known ozone treatment tank comprises an inlet and an outlet for liquid and defines a flow path including, in succession, a downward flow or diffuser zone including a gas bubble inlet and an upward flow or decay zone, the zones being separated by a wall beneath which there is a gap for the flow between these zones. In conventional such systems, the bubble diffuser or other gas inlet has been positioned at the base of the diffuser zone.